Anaerobic digestion is widely used to dispose of a variety of organic waste products in an aqueous waste stream. To employ anaerobic digestion a number of requirements and parameters must be met and/or controlled. In particular, an aqueous waste stream (or feed stream) of organic substances having a significant chemical oxygen demand (COD) or biological oxygen demand (BOD) serves as a substrate for the digestion and is maintained in an anaerobic environment. The substrate is typically digested by a consortium of anaerobic bacteria/microorganisms which reduce the COD and BOD by utilizing the organic substances in metabolic activity involving cell growth and the production of gaseous products.
Conventional anaerobic processing conditions typically include an anoxic environment, controlled acidity/alkalinity, a carbon source, balanced amounts of nutrients supplying nitrogen and phosphorous, and inorganic compounds supplying trace amounts of essential elements. Other parameters which control the metabolism of the microorganisms include a mesophilic temperature range (30-38.degree. C.), a hydraulic retention time (also referred to as a hydraulic detention time) of 7-30 days, a control of acidity in the range of pH 6.5-8.5 with an optimum in the range of pH 6.8-7.2, alkalinity in the range of 1,000 to 5,000 mg/L as CaCO.sub.3, controlled mixing to maintain the bacteria in suspension, and maintenance of the microenvironment clear of secreted metabolic products.
One major problem confronted in conventional anaerobic processes is the elimination of nitrate from aqueous systems. Waste streams containing high concentrations of nitrate and nitrite ions are a serious threat to the ecological balances existing in nature. The discharge of conventionally treated waste effluent into the surface waters of lakes and streams often causes accelerated eutrophication of the waters, since these effluents contain quantities of nitrogen and phosphorus which can promote excessive algae production. Aside from being a major nutrient for algae production, nitrogen in the form of ammonia is toxic to aquatic life and can react with chlorine to form chloramines which are toxic to certain fishes. Water for livestock is considered unsafe at nitrate nitrogen concentrations exceeding 10 ppm. These concentrations can cause methemoglopinemia, vitamin A deficiency, loss of milk production, thyroid disturbances and reproductive difficulties. Nitrite wastes are considered to be injurious to several species of fish at concentrations on the order of 5 ppm nitrite nitrogen. Complete denitrification (conversion of nitrate or nitrite to elemental nitrogen gas) prior to releasing wastes to surface waters is thus desirable.
Nitrates that are present at the low levels found in municipal and agricultural wastes can be removed by conventional processing, but this is not sufficient for handling high concentrations of nitrate wastes. Little effort has been directed towards removing nitrates from wasterwater streams in which the nitrate nitrogen concentration exceeds 500 ppm. Fertilizer manufacturing operations are an example of an operation that generates wastewater streams containing nitrates at these levels.
The best technology reported in the literature for the removal of nitrate is the combined nitrification-denitrification single sludge system described by Tchabenoglous, G., and F. L. Benton, Wastewater Engineering, Treatment, Disposal and Use, Third Edition, Eliason et al., eds., Metcalf and Edg., Inc., McGraw-Hill, Inc., New York, pp. 714-720 (1991). In this process, different zones are maintained in a conventional aerobic activated sludge digestion. In the aerobic process a high productivity of cell mass takes place, which must then be treated by anaerobic digestion as a means of reducing mass of refractory organic material to be disposed. This process is costly and requires multiple stages.
What is needed in the art is a new method for the anaerobic digestion of aqueous waste streams in which nitrate ion can be efficiently removed in a single stage, batch-fed, continuous digester, eliinating the need for multiple-stage treatment. Surprisingly, the present invention provides such a method.